א וְאֵלֶּה הַגּוֹיִם אֲשֶׁר הִנִּיחַ ה' לְנַסּוֹת בָּם אֶת יִשְׂרָאֵל אֵת כָּל אֲשֶׁר לֹא יָדְעוּ אֵת כָּל מִלְחֲמוֹת כְּנָעַן

These are the nations that the Lord left and did not destroy entirely during the days of Joshua, in order to test Israel with them. In principle, Israel could have defeated all of the nations of the land, but they failed to do so, sometimes for lack of desire, at other times for lack of ability. God allowed those nations to survive in order to test all those who did not know all the wars of Canaan;

ב רַק לְמַעַן דַּעַת דֹּרוֹת בְּנֵי יִשְׂרָאֵל לְלַמְּדָם מִלְחָמָה רַק אֲשֶׁר לְפָנִים לֹא יְדָעוּם

it was only so that the generations of the children of Israel would know, to teach them war. The purpose of the continued presence of the enemies was to instruct Israel how to fight wars, since they did not know of it beforehand. Because they had no prior experience in battle, they therefore preferred to enact peace treaties rather than fight. The early commentaries explain the verse differently: The later generations needed to familiarize themselves with war due to their sins, unlike their more worthy predecessors, for whom miracles were performed.